1. Field
The present disclosure relates generally to oscillator devices and, more specifically, to an oscillator circuit for measuring signal propagation delay through integrated circuits.
2. Background
A ring oscillator is typically considered to be a device composed of an odd number of inverters whose output oscillates between two voltage levels. The inverters are connected in a ring, and the output of the last inverter is fed back to the input of the first inverter. Each inverter outputs an inverted voltage level a finite amount of time after the input voltage level of that inverter has changed. Thus, each inverter acts as a delay element. The feedback of an inverted voltage to the input of the first inverter causes the voltage levels in the ring to oscillate. The frequency of oscillation depends on the delay of each inverter and the number of inverters in the ring. In a typical implementation of a delay element, the delay decreases as the voltage that powers the delay element increases. Therefore, one common application of a ring oscillator is in a voltage-controlled oscillator (VCO).
FIG. 1 (prior art) shows a conventional ring oscillator 10 formed from five inverters. Conventional wisdom has held that a ring composed of an even number of ring elements cannot oscillate because the output voltage level of the last ring element would be the same as the original input voltage level of the first ring element. Ring oscillators with an even number of ring elements, however, have been developed. For example, FIG. 2 (prior art) shows a ring oscillator 11 formed from an even number of ring elements that is connected in a differential arrangement. Each inverter in the ring is driven by differential signals.
Another application of ring oscillators is to test signal propagation delay in integrated circuits. Faster switching integrated circuits provide benefits, especially in the fields of communications and data processing. In order fully to benefit from the faster switching speeds, however, the limits of the performance of the integrated circuit must be determined. Ring oscillators can be used to test the speed performance of a circuit design.
FIG. 3 (prior art) shows a ring oscillator 12 formed from an even number of ring elements that is used to test speed performance of an integrated circuit. The frequency of ring oscillator 12 is used to measure how long a test signal takes to propagate through the test integrated circuit. The propagation delay measured by ring oscillator 12 is the sum of delays through various components, including two flip-flops, two delay elements and an AND gate. The frequency of the oscillation of the ring oscillator 12 alone does not indicate the clock-to-out delay of one of the flip-flops, the delay of a delay element, or the delay of the AND gate. Other methods must be used to determine these individual delays.
A method is sought for using a ring oscillator to determine the signal propagation delay through a distinct component type, such as a multiplexer. Moreover, it can be difficult to measure propagation delay along a signal path that passes through the select input of a multiplexer. Thus, a method is sought for determining the select-to-output delay of a multiplexer.